Nonferrous propeller blade retention



- Feb 6, 1945- A c. R. GAsKELL ETAL 2,368,656 i NONFERROUS PROPELLERBLADE RETENTION s: Il

LL-. L TM l /Nvc/v rar: ne v//v 6l Glass/v Feb. 6, 1945. c. R. GASKELL.-mm.v

Filed April 14, 1943 5 Sheets-Sheet 2 (FF-ign Feb. 6, 1945. c. R.GASKELL ET A1. 2,368,656

NONFERROUS PROPELLER BLADEv RETENTION Filed April 14, 1943 5Sheets-She'et `3v 1 y "H lll l r l 'l' nl l), A" I 11 l \\\\Y7//// El! lall M# l/ W i; i? i l, .f'il a`I m ,M

Feb. 6, E945. Q GASKELL, ET AL, 2,368,656

NONFERROUS PROPELLER BLADE RETENTION Feb. 6, 1945. c. RQGASKELL ET Al. l2,368,656

NONFRROUS PROPELLER BLADE RETENTION Filed April 14, 1943 5 sheets-sheet5 rrok/vixs' Patented Feb. 6, 1945 UNITED STATES PATENT OFFICENONFERROUS PROPELLERHBLADE RETENTION CliiiordR. Gaskell and Marvin C.Green, Dayton, Ohio Application April 14, 1943, serial No'. 483,006

(Granted under the ser 6r March 3, 1883, as

amended April 30, l1928; 370 0. G.' 757) Claims.

be separately attached to a hub structure, a greatl amount of diilicultyhas been encountered indevising means for anchoring the blade shanksinto the hubs. The diillculty has `arisen becauseof the low shearstrength and high notch sensitivity of wood and similar materials and ithas been customary to increase thediameter ofthe shank of the blade toincrease the shear area, and hence, in general, the blades are notinterchangeable with metal blades.

It has been proposed in the art to imbedtension rods of steel or thelike in a wood or plastic propeller blade, centrifugal stresses in theblade being transmitted to the rods by bearing washers or the likeimbedded in the blade, but this 'pro-v posal raises many constructionaldiillculties and because of the low bearing strength of wood wouldrequire a large number of rods which would seriously interfere withthetensile strength of the blade. y

It has also ibeen proposed in British Patent 512,152 to Jablonsky tosecure blades made of wood to the propeller hub structure by means ofyscrews passing through a shallow socket-like mounting on the hub andsecured into the wood at an angle such that the axes of the screws lieon the sides of a cone having its apex `located on the longitudinal axis`oi? the blade. This construction, while an advance over priorproposals, is limited to a single circle of screws in order to avoidpiercing the outer fibres and further, the screws must be of largediameter in order to resist the shear and bending stresses exertedthereon which lowers the joint eillciency in terms of the tensilestrength of the root of the blade.

It has further been proposed in the prior art t0 externally thread theshanks of wooden propeller blades and force each blade into acorrespondingly threaded ferrule and to rely upon the external threadson the blade shank `for transmitting the blade stresses into the ferruleand from thence to the'propeller hub structure. Innumerable attemptsalong this llne have been made, and in some of which the plane ofportions of the threads have been offset to furthery increase the areaof shear in the wood, but the difliculty encountered with such astructure has been that it is very diillcultV to maintain machiningtolerances sulciently low tofeilect uniform bearing on all contact areasto insure a tight flt between the threads in' the wood blade and themetal ferrule particularly at low temperatures and further the sheararea in the blade material is very limited such that it is impossible todevelop shear stresses in the Vblade attachment ,means anywhere nearapproaching the ultimate tensile strength of the blade shank, and fromtests made by thel applicants, blade retaining means in accordance withthe invention, having blade shanks of considerably smaller diameter thanemployed in threaded ferrule constructions of which we are aware, havedeveloped more than twice the ultimate strength of such threaded ferruleconstructions.

In accordance with the present invention, screws threaded into the shankof the blade and arranged transmit the tension stresses in the blade,due to centrifugal force, to a mounting plate which is integral with orsecured to the propeller hub structure.

structure and secured in ferrules by a press nt,

which Vferrules transmit a, major part of the bending stresses into thehub structure. By "use 'oi'y a large number of screws it becomespossible to secure a. very large area of the wood in shear' and theshear stress per screw may be made to equal the tensile strength of thescrew. The screws are arranged so that their axes are parallel with orslightly inclined to the longitudinal axis of the blade and are soarranged that a substantlalpart of the tensile stresses are takenthrough material at the center of the blade shank thus greatlyincreasing the ability of the blade to withstand fatigue stresses.

Blade retention in vaccordance-with the'present inventionl makes itpossible to construct pro' peller blades of wood, compressed wood. orplastic f in substantially concentric circles` The blades are socketedin the hub tudinal axis of the blade, and having means for transmittingtension stress from the screws to the hub structure.

It is a further object of the invention to secure a non-ferrouspropeller blade into a propeller hub structure having a blade socket, byprovidinga plurality of screws extending into the blade shanksubstantially parallel with the blade axis, the screws transmittingtensile stress from the blade to the hub, and providing means such as aferrule pressed on the blade shank, or by direct engagement of the bladeshank with the hub socket, for transmitting bending stresses from theblades to the hub structure. l

It is a further object of the invention to provide a propeller bladeretaining means of the character described, including screws threadedinto the shank of the blade along axes substantially parallel with thelongitudinalv axis of the blade and the screws transmitting tensilestresses from the blade to the hub structure, and means being providedfor preloading the screws such that the distribution of tensile loadingamong the screws'will not be appreciably changed by deformations of thecomponent parts of the retention means when normal blade loading isapplied, and further to ensure that the blade material willl engage thescrew threads'throughout their length on the sides of the thread whichtransmit the loads from the blade to the screws.

Other objectsv and features of the invention will become apparent byreference to the detailed description hereinafter given and to theappended drawings in which: Figure 1 is a fragmentary top plan view insection, of the application of the invention to a twobladetwo-position,` controllable pitch propeller;

Figure 2 is a view taken on line 2-2 of Figure 1;

Figure 3 is a fragmentary sectional view illus-` trating to anexaggerated scale the clearance provisions for preloading the bladeretaining screws;l

' Figure 4 illustrates to an enlarged scale the details of aself-tapping screw which is of general application, but particularlyadapted for use in the invention with respect to its use with propellerblades made of compressed or syntheticv resin treated wood;

rFigure 5 illustrates the application of the in` vention to aconventional type two-blade adjustable pitch propeller;

Figure 6 is a view taken on line 8 6 of Figure 5; 1

Figure 7 is a view of a construction similar to Figure 1, showing themanner in which the propeller blade airfoil section is continued intothe propeller hub;

Figure 8 is a sectional view illustrating the application of theinvention to retaining aluminum alloy blades, and; v

Figure 9 is a view taken on line 9-9 of Figure 8.

Referring now to Figure 1, the reference numeral I indicates a.conventional propeller hub 1 of the character such as employed in thewellknown Hamilton Standard two-position controllable pitch propeller inwhich counterweights 2 are eective to vary the pitch of the propellershown) operatively connected to the counterweight is effective forchanging the pitch of the blades in the opposite direction. Since theconl blades in one direction and hydraulic uid presl sure meansincluding a piston member (not struction of such propeller pitchchanging mechanism is well known in the art, the hub structure l hasmerely been indicated by phantom lines. A

central spider 3 also shown in phantom outline is employed to transmitthe torque from the engine crankshaft to the propeller. The abovestructure being old in the art, and since the present invention relatesto a means for securing propeller blades into the hub structure pr thelike, only those parts more immediately concerned with the inventionhave been shown in full lines and will now be described.

A sleeve 4 having bearings 5 and 6 is mounted ori-the spider 3 so thattorque is transmitted from the spider to the sleeve 4, the sleeve beingclosed at its outer end to prevent leakage of oil from the hydraulicmechanism and is provided with the conventional inwardly directedthreaded stud 1, on which is adjustably threaded a nut 8 for adjustingthe .balance of the propeller assembly. The sleeve 4 at its inner end isprovided with an annular ilange portion 8 which abuts the inner end of atubular ferrule I0 which is threaded at I'I to receive a clamping nutI2, the latter having a lip portion I3 adapted to engage the upper sideof the flange 8 and retain the sleeve 4 in fixed relation with respectto the ferrule I 8 and nut I2. A conventional ring member vI4 showninphantom, interconnects the counterweight 2 to the nut member I2 and isadjustable about the longitudinal axis ofthe propeller and fixed bymeans of taper pins, and serves to transmit turning torque from thecounterweight or hydraulic fluid pressure actuated means to the nut I2and sleeve 4, the sleeve, ferrule and nut being further securely lockedby means of a plurality of taper pins such as indicated at I5. 4A thrustbearing generally indicated by the reference numeral I6 is interposedbetween the bottom 'of the nut I2 and the inner face of-the hub I andserves to transmit centrifugal tension from the propeller blades throughthe ferrule I0 into the hub structure. l Y

The ferrule I8, midway between itsends, is provided with an inwardlyprojecting annular flange I8 through which a plurality of locating pinssuch as I9 and 20 project, which pins serve to angularly locate adisk-like plate member 22 which abuts the end face of the annular ilangeI8` theplate member 22 preferably being brazed into place to form apermanent assembly after the same has been properly located with respectto the ferrule I8. .The plate member 22 is pierced by a plurality ofclearance holes 23 arrangedin the form of concentric circles (see Fig.2) and through which respectively passA a number of screws 25, which arescrewed into the shank of the propeller blade to secure the sameintozthe ferrule. The ferrule I0 at its outer end is counterbored as at28 to form a socket which is provided With a Vconical seat as at 28 solas to receive theV circular shank 3I of aA propeller yblade k30 whichmay be constructed for example of laminatedv wood, syntheticresin-impregnated compressed wood known in the art as Compreg, orimpregnated fabric known as Micarta, or other similar compositions.

The shank 3| of the propeller blade is pressed into the counterbore orsocket 28 in .the ferrule I0 such that a' considerable radial stress isvexerted on the blade shank, and this serves to transmit bending loadsfrom the propeller .blade into the ferrule, from whence the same aretransmitted into the yhub structure through the thrust bearing. Theinner end face 32 of the blade shank 3l abuts the plate member 22 withthe exception of the clearance space -33 (note Fig. 3) which is providedbyrecessing the face of thev plate member 22. The shank of the propellerblade is jig drilled tapping size as indicated at 34`for each of therespective screws and the bottomof each of the drilled lholes is roundedas at to prevent the concentration of stress. The screws 25 which arepreferably of the self-tapping vtype are screwed into the holes. afterthe blade has been pressed into the ferrule socket 28 and the screws aredrawn up unti1 the respective screws have received apredeterminedtensile preload, measured for example by a f torquevindicating wrench. When a tensile load isv applied to a blade withretention means as shown in Figure 1 there is a tendency for thedeformation 'of the plate member22 to cause the screws-near the axis ofthe blade to carry less load than the screws near the periphery of theplate. A .predetermined-load distribution among thevscrews can beeffected by causing an initial deformation of the plate member 22 whichis accomplished bypreloading the screws; that is, the'screwsare drawnupto a total tensile load near the normal tensile loading of the bladewhich causes plate 22` to deect into the clearance space 33. f As thescrews are tapped into the material of the blade, they have a tendencyto bear on the outer face of the threads and in order to insure that thescrew threads will be in contact on their inner faces with the materialof the fblade shank throughout their length, it is y necessary topreloadthe screws, which is accomplished `by means of the clearancespace 33 previously noted; that is, as thescrews are tightened the woodin the -blade shank is elongated and this insures that the threads ofthe respective screws will be in contact throughout vtheir length withthe material of the blade shank. The amount of clearance 33 is generallyof the order of about ten one-thousandths of an inch.

It will be noted by reference to Figure 2 thatr circles decrease fromthecenterV outward; that is,

the length of each respective circle of screws moving outward isprogressively shortened 'and this is done to effect higher strength inthe retention means by transferring a portion ofthe total load from theblade material to the longer screws which results in a'lower meantensile stress in the blade material at the cross sectionVwhere theareaof the blade material has been-'reduced further by the drilled holes atsaid cross section. This is analogous to the common practice -in thedesign of a triple riveted butt joint wherein the spacing of the outerrow of rivets is greater than that'of the adjacent row..

Tension loads from the b1ade'30 are transmitted to the screws 25 bymeans of lshear stress in the wood, and the screws are' so designed thatthe strength of the screw in shear in thewood is'substantially equal toor exceeds the tensile strength of the screw. For example, a screwhaving a root diameter of. one-fourthl inch, and anv outside diameterofjlve-sixteenths of'an inch can carry about 6,000. lbs. load for anoverall length of thread of three inches in a material having` a shear-str ength of approximately 2,000 lbs. per square inch, which isapproximately the ultimate strength of such a screw in tension when madefromk a material vhaving an ultimate strength of 125,000 lbs. per squareinch. It is thus seen'that it is 'easily'possible by means of a largenumber of screws passing into the blade shank to devel-op, by 'shearstress in the wood, loads in the y screws'25 which will substantiallyequal the ultimate tensile strength of the blade shank, and actual testshave shown that the efficiency of. the Joint, determined 'as the ratioof the strength in shear* of the bolts in the wood, to the ultimatetensile' strength of thenet sectionoithe blade has consistentlyfallenwithin the neighborhood of `%',vand in numerous tests the bladeshavefailed in tension rather vthan by shearing out of the screws. i

be exerted on the blade shank to' prevent any local cracking of'therblade shank due to side components of force from the screw threadswhich would otherwise 4tend to produce radial stresses on the bladeshank, and failure from this cause is effectivelyv prevented by pressingthe blade the thread length 'of the screws located at `points radiallyoutward, insuresA that a maximum `transmitted through the `woodvadjacent the cen-v ter of the blade shank at a pointwhere bendingstresses are low',V and this protects the -blade against failure fromfatigue loading whichlisfal- Ways mostfsjevere at the outer fibres wherealter# nating stresses are ata maximum In this regard it will beyfurther noted. by reference to Figure 1 that ample material Visprovided between l the outermost row offscrews 25 and the periphery ofthe blade shank so that the'material in the shank subjected to heavybending stresses is not required to carry shear load, and further thereduction of area `ofthe net section ofgthe blade shank is positionedat. a point where its eiect on `bending stresses will be a minimum. l

By means of the novel blade retain'ingw'means such as contemplated bythe inventionit becomes possible to reduce the .shank 'diameter of woodblades so that suchbladesk can be employed to replace metal blades inthevarious adjustable pitch, controllable pitch and automatic variablepitch prope-llers nowsowidely employed Ain the art. It will be furthernoted that when using materials other than wood, the length of tbescrews 25 may be altered from. that asy shown in Figure 1 to takeadvantage ofthe` largersheal strength` available insuch other `materialsas Compregand thosepreviously mentioned.V g While it is entirelypossible to employ tapsto 0 perform the threads in the Wood ornon-ferrous material of the blades, it has beeniound` in prac- 0indicated by the reference numeral 4'0 has ashank threads per inch, andthe thread angles are such 5, that the rear face of the threads isinclined at an I It is important thatsuiiicient4 radial pressure amountof tensile stress from the blade.Y will be seen in this figure. Thescrew which is generally angie of *5 to the vertical and the freni faceof the threads is inclined at an angle of to the s vertical and theincluded angleof the threads is lThe threads extend substantially to theouterend of the screw, and in order to facilitate self-tapping, thescrew is ground oil from the root diameter on a slope ofy approximately10 which leaves a tapered flat face on the screw which serves as areamer and further allows space for the accumulation of chips, and thisreamer face has been found to greatly facilitate the tapping of thesescrews into material such aslcompressed wood. The screws such asillustrated in Figurev 4V form no part of the present invention, but

' forms the subject matter of copending application Serial Number483,007 led April` 14, ,1943 ln the vname of Clifford R. Gaskell, now U.S. Patent No.

2,350,346 granted June 6, 1944.

The application of the invention to a, conventional two-blade adjustablepitch, propeller is illustrate-dV in Figures 5 and 6 and referring toFigure 5 the propeller hub la is of well-known` type, split along thelongitudinal axis of the propellervblades and adapted to be held inassembled` relation by means of external clamps not shown.`

The hub is counterbored as at 50. to provide a socket for receiving theshank 37| of the propeller blade V30, -which may be for example, made ofmaterial similar to that as noted with respect to Fig. 1.4 The hub isfurther counterbored as at 52 andhas a thrust ange 53 formed by theupper f wall of the counterbore 52. A platemember 22` similar to that ofFigure 1 is housed within the counterbore 52 and bears against thethrust face 53 to transmit the propeller blade tension stresses into thepropeller hub. The plate 22j' has integrally formed therewith a shortferrule IUa into which'a necked downportionla of the propeller shankr3lis pressed, the ferruleserving to prevent radial fracture of thepropeller blade shank due to side stresses from the screw threadsz asdescribe-dl with reference tol Figure 1. platey22 is secured in abuttingrelation to the end The face 32 of the propeller blade 3Q by screws 25similar to blade retention means describedwith-A reference to Figure 1and the screws being ar` ranged in concentric circles as noted inFigurel l6. Balancing holes v55 are provided for securing verticalbalance of the blades and a similar centrally located balancing hole 56is provided for securing vhorizontal balancev ofV the vrespectiveblades. f Upon assembly the blade shank is pressed into'the ferrule IUaand the screws 25 driven into the blade shank and the two halves of thehubare then assembled around the blades and clamped by means not; shown,the blades being adjusted about their longitudinal axis to obtain thedesired blade angle prior `to nal` clamping, the ferrule |0a and plate22 rotating as a unit withV the blade during pitch adjustment. 'I'hetransmission of stresses from the blade through the screws 25 and plate22 to the hub .v la is identical tothefunctioningof the blade retentiondescribed with respect to Figure 1.

Figure 7 shows a propeller hub and blade re.

taining means substantially identical with that.l

previously described with respect to Figure 1, and Vdiler'ing therefromonly in that the alrfoil section of thepropeller blade 30 is retainedsubstantially to the upper portion' of the blade fer rule l 0 whichpermits the blade portion adjacent the hub to assist in engine cooling,this construction of course being only desirable where cowling lclearances permit.

Figures 8 and 9 illustrate the applicationofl the invention to retainingaluminum or magnesium alloyblades in a conventional hub structurevsimilar forexampleto that employed in conjunction .with the, Curtisselectric variable pitch propeller, vand in this construction a ierrule60 vis-counterbored at its outer end as at 6l to provide a socket forreceiving thev inner end of the shank 62 of a propeller blade 63madefrom aluminumalloy. The ferrule 60 .-isl internally threaded as at64 to receive an externallyv threaded plate member 65 which isthreadably apertured as atl 66 to receive a plurality of screws 61equally spaced `on a circle as seen in Figure 9, the screws 61 beingthreaded into tapped holes 68 which extend parallel to the longitudinalaxis of the propellerl blade 53. The screws -61 when` tightenedsecurelyclamp the blade root into the socket 6I and transmit all tensionstresses from the blade intothe platemember 65 from whence they aretransmitted into-the ferrule 60. Plate member 65 is provided with acenter bore '69 through which is passed a long retaining screw- 10threaded at itsouter end into a tapped hole 1l in the propeller bladeshank and provided with an enlargedl head 12 on itsvinner end whichseatsin a counterbore 13 of an annular flange member 15 whichis threadedexternally as at 14 into ,a complementary internally threaded portion 16of rthe ferrule 60.. The member 15 is formed over ay sector thereofwithv gear teeth 18 adapted to mesh with a bull gear,not shown,

through which pitch adjustment of the propeller blades is accomplished.VThe flange member 15 is vprovided with a thrust face -19 ,adapted toengage the inner end ofa thrust bearing assembly, not shown, 'whichVsurrounds the -ferrule 60 and transmits stresses into Y.the hubstructure, not

shown. y;

Except for the difference material, the ,design of .the blade retentionmeans of Figures 8 and9 is similar to that of `the previouslyA describedembodiments and has the advantage that itis not necessary to formcollars, threads, or the like on the periphery' of the blade shankinorder to secure the bla-des into the propellervhub structure, Awhichis a decided advantage in that it eliminates the possibility of stressconcentrations .at the outermost bres of the blade shank and furthereliminates the necessityfor'employing split thrust 'bearing vassembliesas' heretofore commonly employed inthe art. It will be apparent fromconsideration of our invention that thenovel feature of the same ischaracterized bythe employment of a plurality of screws extending intothe blade shank on lines substantially parallel to the longitudinal axisof the blade, which screws are adaptedto transmit tension stresses fromthe blade by shear into a plate member or they like from whence thestresses are transmitted into the hub structure, the major portion ofthe bending loads on the propellerA blades being adapted to be directlyY stresses on the bladeis transmitted into thehub structure throughmaterial adjacent the center portion of the blade shank, which materialis subjected' to lowerv alternating stresses, also the provision forvariation in depths that the drilled holes and screws extend into theblade shank `results in low mean stresses in the blade matethe screws.For purposes of design it is preferable that an optimum size of screw beemployed such that shear stress may be developed by the screws in theblade material approaching or excecding the tensile strength of thescrews, and we have found that screws having a diameter of one-fourth ofan inch over the threads is satisfactory for general use in wood, thescrew maferial being such that tensile stresses of 160,000 to180,0001bs. per square inch may be developed. It is however entirelypossible to use larger diameter screws adjacent the center of the hubshank and smaller diameter screws in the outer row, which makes itpossible to form one or more of such large diameter screws as hollowscrews which may be plugged with lead for balancing purposes. It isfurther obvious that the screws may be inclined inward or outward at aslight angle if desired without departing from the spirit of theinvention.

While preferred embodiments of the invention have been illustrated anddescribed, other varia, tions and changes therein will become apparentto those skilled in the art as falling within theA scope of theinvention as defined in the appended claims.

We claim:

1. Means for anchoring an individual non-fer.- rous propeller blade intoa propeller hub having a blade receiving socket, comprising thecombination with said blade and hub socket, of a ferrule secured in saidhub socket so as to transmit loads to the propeller blade hub, saidpropeller blade having a shank adapted to be tightly received in saidferrule so as to transmit substantially all of the bending loads on thepropeller blade to the ferrule, a transverse plate element associatedAwith said ferrule radially inwardly of the outermost portion of saidferrule and abutting the blade'shank, said plate element being adaptedto transmit tensile stresses from the blade to said ferrule, and aplurality of threaded fastening elements extending into the propellerblade shank along lines substantially parallel with the longitudinalaxis of the propeller blade, said fastening elements interconnectingsaid plate element and said-propeller blade shank and being thesolemeans retaining the blade shank in the ferrule and for transmittingtension loads on said blade to said plate, said fastening elements eachhaving threaded engagement with said propeller blade shank for asubstantial length of said fastening element, said fastening elementsbeing of sumcient length to transmit substantially all the tension loadson the blade to said fastening elements.

2. Means for retaining a non-ferrous propeller blade in a propeller` hubhaving a blade receiving socket, comprising in combination with saidblade. a transverse plate member arranged in said hub socket andabutting the blade and adapted to transmit tension loads to the hub,said propeller blade having a shank adapted to be received in saidsocket, means for transmitting substantially all of the bending stressesfrom said blade shank to said hub structure, a plurality of threadedfastening elements extending into the propeller blade shank along linessubstantially parallel with the longitudinal axis of the propellerblade, said fastening elements interconnecting said plate element andsaid propeller blade shank and being the sole means for retaining theblade in the hub socket and for transmitting tension loads on said bladeto said plate, said fastening elements each having threaded engagementwith said propeller blade shank for a substantial length of saidfastening element, said fastening elements being of sufficient length totransmit substantially all the tension loads on the blade' to saidfastening elements.

3. The structure as claimed in claim 2, in which the propeller bladeshank has an outer marginal portion of its transverse end face in directcontact with said plate member with an initial clearance between theportion of said end face adjacent said fastening elements and said platemember, preloading of said fastening elements deforming said blade shankuntil the entire end face of said blade shank is in contact with saidplate member.

4. The structure as claimed in claim 2 wherein the fastening elementsadjacent the central longitudinal axis of the blade shank are longerthan the fastening elements lpositioned radially outward therefrom,whereby a substantial portion of the tension load on the blade istransmitted transverse adjustably through fastening elements positionedadjacent said central longitudinal axis of the blade shank.

5. The structure as claimed in claim 2, in which the propeller hub isformed of complementary halves adapted to be clamped together along aradial plane containing the propeller blade axes, an annular shoulderwithin said hub forming a thrust abutment and having a diameter greaterthan the diameter of the blade receiving socket and positioned at theradially inward end thereof and concentric with the blade shank axis,said plate member being in the form of an rotatable disc engaging saidthrust abutment and having aplurality of apertures for passage of saidthreaded fastening elements therethrough. said fastening elements beingoperatively connected to said disc for transmission of all tension loadson the blade thereto and the transverse end face of the blade shankenlllng,

